Position-and-velocity sensorless control for starter generator electrical system using generator back-EMF voltage

ABSTRACT

A detector for detecting rotor position of a brushless generator having a motive power shaft, a permanent magnet generator (PMG) having a set of armature phase windings and a main generator portion having a set of armature phase windings includes a circuit coupled to the armature phase windings of the main generator portion for deriving a number of interval pulses per revolution of the motive power shaft. A circuit measures time periods between adjacent interval pulses and the measured time periods are converted into an indication of the angular position of the motive power shaft in accordance with a parameter of power delivered to the main generator portion armature phase windings.

TECHNICAL FIELD

The present invention relates generally to electromagnetic machines, andmore particularly to a detector for detecting the rotor position of abrushless generator and a control system incorporating such a detector.

BACKGROUND ART

An auxiliary power unit (APU) system is often provided on an aircraftand is operable to provide auxiliary and/or emergency power to one ormore aircraft loads. In conventional APU systems, a dedicated startermotor is operated during a starting sequence to bring a gas turbineengine up to self-sustaining speed, following which the engine isaccelerated to operating speed. Once this condition is reached, abrushless, synchronous generator is coupled to and driven by the gasturbine engine during operation in a starting mode whereupon thegenerator develops electrical power.

As is known, an electromagnetic machine may be operated as a motor toconvert electrical power into motive power. Thus, in those applicationswhere a source of motive power is required for engine starting, such asin an APU system, it is possible to dispense with the need for thededicated starter motor and operate the generator as a motor during thestarting sequence to accelerate the engine to self-sustaining speed.This capability is particularly advantageous in aircraft applicationswhere size and weight must be held to a minimum.

The use of a generator in starting and generating modes in an aircraftapplication has been realized in a variable-speed, constant-frequency(VSCF) power generating system. In such a system a brushless,three-phase synchronous generator operates in the generating mode toconvert variable-speed motive power supplied by a prime mover intovariable-frequency AC power. The variable-frequency power is rectifiedand provided over a DC link to a controllable static inverter. Theinverter is operated to produce constant-frequency AC power, which isthen supplied over a load bus to one or more loads.

The generator of such a VSCF system is operated as a motor in thestarting mode to convert electrical power supplied by an external ACpower source into motive power which is provided to the prime mover tobring it up to self-sustaining speed. In the case of a brushless,synchronous generator including a permanent magnet generator (PMG), anexciter portion and a main generator portion mounted on a common shaft,it has been known to provide power at a controlled voltage and frequencyto the armature windings of the main generator portion and to providefield current to the main generator portion field windings via theexciter portion so that the motive power may be developed. This has beenaccomplished in the past, for example, using two separate inverters, oneto provide power to the main generator portion armature windings and theother to provide power to the exciter portion. Thereafter, operation inthe generating mode may commence whereupon DC power is provided to theexciter field winding.

In order to properly accelerate the generator and prime mover duringoperation in the starting mode, it is necessary to properly commutate orswitch the currents among the armature windings of the generator. In thepast, proper commutation was achieved using an absolute position sensor,such as a resolver, a synchro, an optical encoder or hall effectdevices. For example, in Lafuze, U.S. Pat. No. 3,902,073 three Hallsensors are mounted in an air gap of a PMG 120 electrical degrees apartwith respect to permanent magnet rotor pole pairs. As the rotor of thePMG rotates, the voltage output of each Hall sensor switches on and offas a function of the rotor position thereby generating three square wavevoltages spaced apart by 120 electrical degrees. The outputs from theHall sensors are representative of the position of the PMG rotor. Theoutput signals from the Hall sensors are used to control switchingelements in cycloconverters to switch current to armature windings of amain generator portion.

Use of an external absolute position sensor entails considerable expensein the position sensor itself and associated electronics, and furtherresults in extra wires and the need for an extra assembly step toinstall the components. Also, operational parameters often limit theaccuracy of the sensor.

In view of the foregoing difficulties, other approaches have been takenin an effort to detect rotor position without the need for absoluteposition sensors.

In the case of a brushless DC motor control, a back EMF approach hasbeen used to detect rotor position. The back EMF of the motor is definedby the following equation:

    E.sub.emf =Kω Sin α

where K is a constant, ω is the angular speed of the motor and α is theelectrical phase angle of the rotor. From the foregoing equation, it canbe seen that if back EMF can be detected, rotor electrical phase anglecan be determined and thus proper commutation of the armature windingsof the motor can be achieved. The back EMF voltage can be detected usingeither of two methods, referred to as the direct method and the indirectmethod.

The direct method can be used to directly measure phase back EMF voltageonly when the phase winding is not energized by the inverter connectedthereto and when the winding is not short circuited either by closedswitches in the inverter or by conducting flyback diodes in theinverter. Such conditions can be realized when a 120 degree commutationalgorithm is utilized. In this case, a voltage reading is taken after ashort delay following switching of the phase winding off to ensurecomplete current decay by the free-willing diodes. This direct techniqueis described in a paper entitled "Microcomputer Control for SensorlessBrushless Motor" by E. Iizuka et al., IEEE Transactions on IndustryApplication, Vol. IA-21, No. 4, May/June 1985.

The indirect method is based on estimating the back EMF from the motiveterminal voltage and phase currents. This method is suitable for both120 and 180 degree commutation algorithms. One technique that uses thismethod is described in a paper entitled "Position - and - VelocitySensorless Control for Brushless DC Motor Using an Adaptive Sliding ModeObserver" by Furuhashi et al., IEEE Transactions on IndustrialElectronics, Vol. 39, No. 2, April 1992.

Because the back EMF voltage of a motor is zero at standstill and thesignal to noise ratio is small at lower speeds, the reliabledetermination of rotor position by detecting back EMF is limited at lowrotor speeds.

A method of using a permanent magnet generator as a position sensor formotor/generator start is described in Stacey U.S. Pat. No. 5,140,245. Astandard brushless generator is equipped with a PMG which is used as anemergency electric power source and as a source of control power duringa normal or generating mode of operation. The PMG develops a multi-phaseoutput which is supplied to a high resolution phase-locked loop having abinary counter which develops an output signal representing shaftposition. This method, however, is limited to the situation where thenumber of PMG rotor poles is equal to or less than the number of poleson the main generator portion rotor so that ambiguous position readingsare avoided.

SUMMARY OF THE INVENTION

In accordance with the present invention, a detector for detecting rotorposition of brushless generator utilizes inexpensive components andoperates in a simple and effective manner.

More particularly, a detector for detecting rotor position of abrushless generator having a motive power shaft and a main generatorportion having a set of armature phase windings capable of receiving ACpower thereon includes means coupled to the armature phase windings forderiving a number of interval pulses relating to the revolution of themotive power shaft from the AC power supplied to the armature phasewindings. Means are coupled to the deriving means for measuring timeperiods between adjacent interval pulses and means are coupled to themeasuring means for converting the measured time periods into anindication of the angular position of the motive power shaft.

Preferably, the measuring means comprises a counter which accumulatesclock pulses during time periods between adjacent interval pulses. Alsopreferably, the counter is periodically reset a certain number of timesduring each revolution of the motive power shaft by the interval pulses.

Still further in accordance with the preferred embodiment, theconverting means includes means for inverting a counter output signaldeveloped by the counter to obtain an indication of the speed of themotive power shaft. The converting means also preferably includes anintegrator coupled to the inverting means which develops the angularposition indication.

In accordance with a further aspect of the present invention, a startingsystem control for operating a brushless generator in a starting mode toconvert electrical power into motive power wherein the brushlessgenerator includes a motive power shaft and a main generator portionhaving a set of armature phase windings which receive AC power duringoperation in the starting mode includes means coupled to the armaturephase windings for deriving a number of interval pulses relating to therevolution of the motive power shaft from the AC power supplied to thearmature phase windings. Means are coupled to the deriving means formeasuring time periods between adjacent interval pulses and means arecoupled to the measuring means for converting the measured time periodsinto indications of the speed and angular position of the motive powershaft. Preferably, means are coupled to the converting means fordelivering AC power to the set of main generator portion armature phasewindings in dependence upon the speed and angular position indications.

In accordance with yet another aspect of the present invention, a methodof detecting rotor position of a brushless generator having a motivepower shaft and a main generator portion having armature phase windingscoupled to an exciter portion includes the steps of deriving a number ofinterval pulses relating to the revolution of the motive power shaftfrom the AC power supplied to the main generator portion armature phasewindings and measuring time periods between adjacent pulses. Themeasured time periods are converted into an indication of the angularposition of the motive power shaft.

The detector of the present invention does not require the use of anabsolute position sensor, nor is it limited to use with any particularcommutation algorithm.

Additional aspects of the invention will be apparent in view of thefollowing detailed description of the preferred embodiment, made withreference to the drawings, a brief description of which is providedbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A comprises a combined block and schematic diagram of a brushless,synchronous generator;

FIG. 1B comprises a block diagram of an APU system together with a startconverter;

FIG. 2 comprises a block diagram illustrating a rotor position detectorand a starting system control according to the present invention;

FIG. 3 illustrates a number of inputs to and outputs from the systemcontrol of FIG. 2;

FIG. 4 comprises a diagram of the three-phase pulse-width modulatedinverter of FIG. 2;

FIG. 5 comprises a block diagram of the rotor position detector of FIG.2;

FIG. 6 illustrates the inputs to and the output from the logic circuitof FIG. 5;

FIG. 7 comprises a block diagram of the speed controller of FIG. 2;

FIG. 8 comprises a block diagram of the DC current estimator of FIG. 2;and

FIG. 9 comprises a circuit diagram of the phase converter of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1A, a brushless, synchronous generator 10 includesa permanent magnet generator (PMG) 12, an exciter portion 14 and a maingenerator portion 16. The generator 10 further includes a motive powershaft 18 interconnecting a rotor 20 of the generator 10 and a primemover 21, such as a gas turbine engine. In a specific application of thepresent invention, the generator 10 and the prime mover 21 together maycomprise an aircraft auxiliary power unit (APU) 22, although the presentinvention is equally useful in other prime mover/generator applications.

The rotor 20 carries one or more permanent magnets 23 which form polesfor the PMG 12. Rotation of the motive power shaft 18 causes relativemovement between the magnetic flux produced by the permanent magnet 23and a set of three-phase PMG armature windings including phase windings24a-24c mounted within a stator 26 of the generator 10.

The exciter portion 14 includes a field winding 28 disposed in thestator 26 and a set of three-phase armature windings 30a-30c disposed onthe rotor 20. A set of rotating rectifiers 32 interconnect the exciterarmature windings 30a-30c and a main generator portion field winding 34also disposed on the rotor 20. Three-phase main generator portionarmature windings 36a-36c are disposed in the stator 26.

During operation in a generating mode, at least one, and preferably allthree of the PMG armature windings 24a-24c are coupled through arectifier and voltage regulator (not shown) to the exciter portion fieldwinding 28. As the motive power shaft 18 is rotated, power produced inthe PMG armature windings 24a-24c is rectified, regulated and deliveredto the field winding 28. AC power is produced in the armature windings30a-30c, rectified by the rotating rectifiers 32 and applied to the maingenerator portion field winding 34. Rotation of the motive power shaft18 and the field winding 34 induces three-phase AC voltages in the maingenerator portion armature windings 36a-36c as is conventional. As seenin FIG. 1B, the AC voltages are supplied through a contactor set 37 toan APU power distribution network 38 and thence to one or more loads(not shown).

Often, it is desirable to use the brushless generator 10 as a motor tobring the prime mover 21 up to self-sustaining speed. This operation isaccomplished by providing electrical power to the main generator portionfield winding 34 via the exciter 14, providing AC power to the maingenerator portion armature windings 36a-36c and suitably commutating thecurrents flowing in the windings 36a-36c to cause the motive power shaft18 to rotate. In a specific embodiment, the electrical power for thegenerator 10 is developed by an APU start converter 39, FIG. 1B, whichreceives external electrical power and which is connected by contactorsets 40a, 40b to the exciter field winding 28 and the armature windings36a-36c, respectively. Various methods have been devised for controllingthe power supplied to the main generator field winding 34 via theexciter 14. Inasmuch as the method of exciter control forms no part ofthe present invention, it will not be described in detail herein.

FIG. 2 illustrates the PMG 12, the main generator portion 16 and themotive power shaft 18 of the generator 10 together with a startingsystem 41 for providing electrical power to the armature windings36a-36c during operation of the generator 10 in the starting mode toconvert electrical power into motive power. While not shown, power mayalso be provided to the exciter field winding 28, and thus to theexciter armature windings 30a-30c and thus to the main generator portionfield windings 34 by any suitable means during operation in the startingmode. The application of power to the exciter field winding 28 forms nopart of the present invention and will not be described in detailherein. The starting system 41 includes a rotor position detector 44which receives via lines 43a-43c the voltages developed on a series oflines 45a-45c coupled to the main generator portion armature windings36a-36c of the main generator portion 16 and three synchronizationsignals developed on a series of lines 46a-46c by the starting system 41as described in detail below. The rotor position detector 44 develops asignal representing the angular position of the motive power shaft 18 ona line 47 and a signal representing the speed of the motive power shaft18 on a line 49.

The speed controller 48 develops a ramp signal on a line 50 representingthe mechanical position of the motive power shaft 18 based on theangular position signal on the line 47. The signal on the line 50 issummed with a phase advance signal by a summer 54. The phase advancesignal is developed by a function generator 56 and is dependent upon thespeed of the motive power shaft 18 as detected by the rotor positiondetector 44. The function generator 56 provides increasing phase advanceas speed increases in a high speed range. The summer 54 develops anelectrical angle command signal on a line 58 which is supplied to firstand second functional blocks 60, 62 which generate a cosine waveformsignal and a sine waveform signal, respectively, each of which has thesame frequency as the electrical angle command signal on the line 58.

The cosine signal and the sine signal are supplied via a pair of lines61, 63 to a 2-to-3 phase converter 64 which converts these signals intothree-phase signals which are in turn supplied to three zero crossingdetectors 66a-66c via lines 67a-c, respectively. Each of the zerocrossing detectors 66a-66c detects the zero crossings of one of thethree-phase signals produced by the phase converter 64 to producesquare-wave commutation signals S_(a), S_(b), and S_(c), respectively,which are shown in FIG. 3 and which are separated in phase by 120°. Acontroller 68 responds to the signals S_(a), S_(b), and S_(c) and to apower control signal delivered on a line 70 to produce on lines 72a-72fpulse-width modulated (PWM) inverter control waveforms T₁, T₃, T₅ andcommutation signals T₂, T₄, T₆ which are shown in FIG. 3.

The controller 68 also produces the synchronization signals on the lines46a-46c. Referring to FIG. 3, the voltage provided to the main generatorportion armature winding 36a is shown as waveform V_(a) ; the currentprovided to the winding 36a is shown as waveform I_(a) ; and thesynchronization signal associated with the winding 36a is shown aswaveform SYNC_(a). The voltage, current and synchronization signalwaveforms associated with the windings 36b and 36c are identical tothose shown for the winding 36a, except that they are phase-shifted by120°. The synchronization signal SYNC_(a) for winding 36a has a highvalue when neither of the inverter control waveforms T₁ and T₂ (whichcontrol TR₁ and TR₂ to control the current provided to the winding 36a)has a high value. Thus, SYNC_(a) has a high value when TR₁ and TR₂ areboth off, which coincides with the period during which the voltage V_(a)makes a zero crossing. The synchronization signals selectively enablethe comparators 102 to detect the phase voltages only when theirassociated windings are not in conduction so that the winding phasevoltage detected is equal to the back EMF voltage. In detecting rotorposition, it is assumed that the rotor 20 moves in accordance with thephase voltages provided to the main generator portion 16 by the inverter74.

Referring again to FIG. 2, the inverter 74 receives DC power over a DClink 76. The DC link 76 may receive DC power from a three-phaserectifier 78 which is in turn coupled to an external AC source 79 or anyother type of DC source.

Referring now to FIG. 4, the inverter 74 includes controllable powerswitches TR₁ -TR₆ and associated flyback diodes D1-D6 connected in athree-phase bridge configuration across DC link conductors 76a, 76b.Referring again to FIG. 3, the control waveforms T₁, T₃ and T₅ includePWM notches in the positive-going positions thereof. These signalscontrol the power switches TR₁, TR₃ and TR₅, respectively, of theinverter 74. The control waveforms T₂, T₄ and T₆ control the inverterpower switches TR₂, TR₄ and TR₆, respectively. The widths of the PWMnotches of the control waveforms T₁, T₃ and T₅ are controlled by a powercontrol circuit 80.

Referring back to FIG. 2, the power control circuit 80 includes a summer82 which subtracts a signal I_(dc) representing the DC link currentprovided on a line 81 (which in turn represents the power delivered bythe inverter 74 to the armature windings 36a-36c) from a power referencesignal. The I_(dc) signal on the line 81 is generated by a DC currentestimator 83 based upon three current signals I_(A), I_(B), and I_(c)generated by three current transformers coupled to the outputs of the ACpower source 79. As shown in FIG. 8, the DC current estimator maycomprise a three-phase rectifier 85 coupled to a conversion factorcircuit 87 such as a multiplier or scaled amplifier.

Referring back to FIG. 2, the power error signal generated by the summer82 is conditioned by a conditioner 84 which, preferably, comprises aproportional-integral type compensator but could, alternatively,comprise any type of gain and compensation unit. The output of theconditioner 84 is delivered to a controlled or adjustable limiter 86which produces the power control signal on the line 70.

The limiter 86 is controlled in accordance with a limiter controlsignal. The speed indication produced by the rotor position detector 44is delivered to a multiplier 88 and multiplied by a constantvolts-per-hertz signal to produce a speed dependent voltage signal. Thespeed dependent voltage signal is summed with a constant boost voltageby a summer 90, which in turn produces the limiter control signal. Theconstant boost voltage signal is proportional to the IR voltage drop inthe main generator portion control windings and allows the power controlsignal to overcome these losses at initial start-up. Thereafter, as thespeed indication developed by the rotor position detector 44 increases,the multiplier 88 produces a ramping signal which, when added to theboost voltage, increases the adjustable limit of the limiter 86 so thatincreasing power magnitudes can be delivered to the armature windings36a-36c by the inverter 74.

Referring now to FIG. 5, the rotor position detector 44 is shown ingreater detail. The voltage waveforms provided to the main generatorarmature windings and sensed via the lines 43a-43c are supplied throughlevel shifting amplifiers 100a-100c and zero crossing detectors102a-102c to a logic circuit 104. Each zero crossing detector 102a-102cis enabled for the period of time during which its respectivesynchronization signal provided on one of the lines 46a-46c has a highvalue.

FIG. 6 illustrates the phase currents I_(a), I_(b), I_(c) and threewaveforms S_(d), S_(e), and S_(f) representing the outputs of the zerocrossing detectors 102a-102c, respectively. From the waveforms S_(d)-S_(f), the logic circuit 104 develops a signal S_(g) consisting ofbrief pulses separated by 60 electrical degrees as shown in FIG. 6.Referring back to FIG. 5, the waveform S_(g) is provided by the logiccircuit 104 to a delay circuit 106. The delay circuit 106 periodicallyprovides a reset signal to a counter 108 which accumulates clock pulsesproduced by a clock 110. The counter 108 is reset every 60° with respectto the waveforms provided to the main generator portion 16, and thusevery 60° of rotation of the motive power shaft 18.

The output of the counter 108 represents the time that elapses betweeneach pulse in the waveform S_(g). The falling edge of each pulsecomprises a write command to a latch 112 which latches the output of thecounter 108. The output of the latch 112 is inverted, i.e. thereciprocal thereof is calculated, by a circuit 114 to generate a speedsignal indicative of the speed of the motive power shaft 18. Since thelatch 112 is activated once every 60° period, the speed signal generatedhas a constant value for each 60° period.

The angular position of the motive power shaft 18, and thus the rotor20, is determined by an integrator 116 comprising a multiplier 116acoupled to an accumulator 116b. During each 60° period, the multiplier116a multiplies the speed signal, which is constant for each period,with the output of the counter 108, which increases during the period,to generate a ramp signal on the line 117. The magnitude of that rampsignal falls to zero every 60° since the counter 108 is reset every 60°.

The periodic signal S_(g) is also provided to a staircase generator 118which generates a staircase signal that increases a constant amount each60° period. The magnitude of the staircase signal at each 60° periodrepresents the rotor position at the start of that period. Every 360°,corresponding to one revolution of the rotor 20, the magnitude of thestaircase signal falls back to zero.

The accumulator 116b continuously determines the rotor position byadding the current magnitude of the ramp signal generated by themultiplier 116a with the magnitude of the staircase signal for thecurrent 60° period. The resulting position signal generated by theaccumulator 116b is a ramp signal having a magnitude representing rotorposition. The period of the ramp signal corresponds with 360°, one fullrevolution of the rotor 20. Other integrative techniques could also beused.

FIG. 7 illustrates the speed controller 48 in greater detail. A speedcommand signal may be developed on a line 130 which is coupled to anon-inverting input of a summer 132. The speed command may comprise astep voltage from a first voltage to a second, higher voltage or maycomprise any other type of waveform as desired. The output of the summer132 is coupled to a function generator 134 which develops anacceleration command signal which is, in turn, integrated by anintegrator 136 to produce a speed reference signal. The speed referencesignal is fed back to an inverting input of the summer 132, and hencethe elements 132, 134, and 136 comprise a closed-loop circuit. The speedreference signal is integrated by a further integrator 138 to develop aposition reference signal which is, in turn, provided to a controllableswitch 140 and to a non-inverting input of a summer 142. The angularposition indication comprising the position signal from the integrator116 of FIG. 5 is also provided to the controllable switch 140 and isfurther provided to an inverting input of the summer 142. The summer 142produces a position error signal indicative of the error between thederived position reference signal and the actual rotor position asdeveloped by the integrator 116. The position error signal is providedto an error comparator 144 which compares the position error signal toan error reference and produces a high state signal on a line 146 whenthe position error signal is less than the error reference.

Furthermore, the speed reference signal produced by the integrator 136is provided to a comparator 150 that compares the speed reference signalwith a valid speed reference signal indicative of the value at which thespeed reference signal becomes a reliable representation of the speed ofthe motive power shaft. If the speed reference signal equals or exceedsthe valid speed reference signal, the comparator 150 produces a highstate output signal on a line 152 at such time. The signals on the lines146 and 152 are delivered to an AND gate 156 having an output which iscoupled to and controls the controllable switch 140.

At the initiation of a start-up sequence, at which time the speed of themotive power shaft 18 is zero, the controllable switch 140 is set to aposition which passes the output of the integrator 138 to the output ofthe switch 140, and thus to the summer 54 of FIG. 2. Also at this time,power is applied to the exciter portion 14, and hence to the maingenerator portion field winding 34 of FIG. 1A, and power is also appliedto the main generator armature windings 36a-36c. The motive power shaft18 is thus accelerated. When the error between the rotor position, asmeasured by the integrator 116 of FIG. 5, and the position referencesignal, as derived by the integrator 138, is less than the errorreference and when the speed reference signal produced by the integrator136 is equal to or greater than the valid speed reference, thehigh-state signals on the lines 146 and 152 cause the AND gate 156 tomove the controllable switch 140 to the position shown in FIG. 7. Thecontrollable switch 140 is latched in such position so that the outputof the accumulator 116b of FIG. 5 is thereafter provided to the summer54 of FIG. 2. The switch 140 remains latched until a new start-upsequence is initialized.

FIG. 9 illustrates in greater detail the phase converter 64 shownschematically in FIG. 2. The converter 64 includes three operationalamplifiers 160, 162, 164 and associated biasing circuitry connected tothe lines 61, 63 which generate outputs on the lines 67a-67c.

Numerous modifications and alternative embodiments of the invention willbe apparent to those skilled in the art in view of the foregoingdescription. Accordingly, this description is to be construed asillustrative only and is for the purpose of teaching those skilled inthe art the best mode of carrying out the invention. The details of thestructure may be varied substantially without departing from the spiritof the invention, and the exclusive use of all modifications which comewithin the scope of the appended claims is reserved.

We claim:
 1. A detector for detecting rotor position of a brushlessgenerator having a motive power shaft and a main generator portionhaving a set of armature phase windings which is supplied a parameter ofAC power, comprising:means coupled to the main generator portionarmature phase windings for deriving a number of interval pulsesrelating to the revolution of the motive power shaft from a parameter ofthe AC power supplied to the main generator portion armature phasewindings; means coupled to the deriving means for measuring time periodsbetween adjacent interval pulses; and means coupled to the measuringmeans for converting the measured time periods into an indication of theangular position of the motive power shaft.
 2. The detector of claim 1,wherein the measuring means comprises a counter which accumulates clockpulses during time periods between adjacent interval pulses.
 3. Thedetector of claim 2, wherein the converting means includes means forinverting a counter output signal developed by the counter to obtain anindication of the speed of the motive power shaft.
 4. The detector ofclaim 3, wherein the converting means further includes an integratorcoupled to the inverting means which develops the angular positionindication.
 5. A starting system control for operating a brushlessgenerator in a starting mode to convert electrical power into motivepower wherein the brushless generator includes a motive power shaft anda main generator portion having a set of armature phase windings whichare supplied a parameter of AC power during operation in the startingmode comprising:means coupled to the main generator portion armaturephase windings for deriving a number of interval pulses relating to therevolution of the motive power shaft from the parameter of AC powersupplied to the main generator armature phase windings; means coupled tothe deriving means for measuring time periods between adjacent intervalpulses; means coupled to the measuring means for converting the measuredtime periods into indications of the speed and angular position of themotive power shaft; and means coupled to the converting means fordelivering AC power to the set of main generator portion armature phasewindings in dependence upon the speed and angular position indications.6. The starting system control of claim 5, wherein the delivering meanscomprises first means for developing an electrical angle command signalfrom the angular position indication, second means for developing apower control signal from the speed indication and an inverterresponsive to the electrical angle command signal and the power controlsignal for delivering the AC power to the set of main generator portionarmature windings.
 7. The starting system control of claim 6, furtherincluding second means for converting the electrical angle commandsignal into a sine signal and a cosine signal and third means forconverting the sine and cosine signals into three-phase signals.
 8. Thestarting system control of claim 7, further including three zerocrossing detectors each responsive to one of the three-phase referencesignals for producing first, second and third inverter control signals,respectively, and an inverter controller responsive to the power controlsignal and coupled between the zero crossing detectors and the inverterwhich operates the inverter in accordance with the first, second andthird inverter control signals and the power control signal.
 9. Thestarting system control of claim 6, wherein the second developing meansincludes a first summer for summing a power reference with a signalindicative of the power produced by the inverter to produce a powererror signal, means for conditioning the power error signal and meansfor limiting the conditioned power error signal to produce the powercontrol signal.
 10. The starting system control of claim 9, wherein thesecond developing means further includes a multiplier for multiplyingthe speed indication by a reference signal to produce a ramp signal anda second summer for summing the ramp signal and a boost signal toproduce a signal for controlling the limiting means.
 11. The startingsystem control of claim 6, wherein the motive power shaft is acceleratedfrom zero speed to a particular speed over a series of revolutions,wherein the angular position indication comprises a position signal, andwherein the first developing means includes a position reference signalgenerator which develops a position reference signal, a switch whichprovides the position reference signal to an output thereof during aninitial portion of the series of revolutions and which provides theposition signal to the output after the initial portion of the series ofrevolutions, and a summer for summing the signal at the output of theswitch with a phase advance command to obtain the electrical anglecommand signal.
 12. The starting system control of claim 11, wherein thefirst developing means further includes third means for detecting when aspeed command signal is equal to or greater than a valid speedreference, fourth means responsive to the position reference signal andthe position signal for developing a position error signal, an errorcomparator for comparing an error reference to the position errorsignal, and means responsive to an output of the detecting means and toan output of the error comparator for controlling the operation of theswitch.
 13. The starting system control of claim 12, wherein thecontrolling means comprises an AND gate.
 14. The starting system controlof claim 5, wherein the measuring means comprises a counter whichaccumulates clock pulses during time periods between adjacent intervalpulses.
 15. The starting system control of claim 14, wherein theconverting means includes means for inverting a counter output signaldeveloped by the counter to obtain the speed indication.
 16. Thestarting system control of claim 15, wherein the converting meansfurther includes an integrator coupled to the inverting means whichdevelops the angular position indication.
 17. An electric powerconversion system operable in a starting mode and in a generating mode,comprising:an exciter portion including an exciter field winding and aset of exciter armature windings, said set of exciter armature windingsbeing coupled to a rotor so as to rotate with said rotor; a maingenerator portion including a main generator field winding coupled tosaid rotor so as to rotate with said rotor, said main generator fieldwinding also being coupled to said exciter armature winding, and a setof main generator armature windings; an inverter for supplying AC powerto said main generator portion when said power conversion system is insaid starting mode; and a rotor position detector coupled to said maingenerator portion, said rotor position detector determining the angularposition of said rotor based upon a parameter of said AC power providedto said main generator portion.
 18. A power conversion system as definedin claim 17 wherein said parameter of AC power comprises voltage.
 19. Apower conversion system as defined in claim 17 wherein said rotorposition detector determines the angular position of said rotor bydetermining the elapsed time between a plurality of zero crossings ofsaid parameter of AC power.
 20. A power conversion system as defined inclaim 17 additionally comprising a controller coupled to said rotorposition detector for generating a position signal, said position signalbeing generated during a first period of time based on a referencesignal and during a second period of time based on said parameter of ACpower.
 21. A power conversion system as defined in claim 20 wherein saidcontroller comprises a speed controller and wherein said referencesignal comprises a speed command signal.
 22. A method of detecting rotorposition of a brushless generator having a motive power shaft and a maingenerator portion having a set of armature phase windings capable ofreceiving AC power thereon and coupled to an exciter portion, the methodcomprising the steps of:deriving a number of interval pulses relating tothe revolution of the motive power shaft from AC power provided to themain generator portion armature phase windings; measuring time periodsbetween adjacent interval pulses; converting the measured time periodsinto an indication of the angular position of the motive power shaft.23. The method of claim 22, in combination with the further step ofdelivering AC power to the set of main generator portion armature phasewindings in dependence upon the angular position indication.
 24. Themethod of claim 22, wherein the step of measuring comprises the step ofaccumulating clock pulses in a counter during time periods betweenadjacent interval pulses.
 25. The method of claim 24, wherein the stepof measuring additionally comprises the step of using the intervalpulses to periodically reset the counter a certain number of timesduring each revolution of the motive power shaft.
 26. The method ofclaim 24, wherein the step of converting additionally comprises the stepof inverting a counter output signal developed by the counter to obtainan indication of the speed of the motive power shaft.
 27. The method ofclaim 26, wherein the step of converting further comprises the step ofproviding an integrator for integrating the inverted counter outputsignal to develop the angular position indication.
 28. The method ofclaim 27, in combination with the further step of delivering AC power tothe set of main generator portion armature phase windings in dependenceupon the angular position indication and the speed indication.